Technology for integrating metals and resins is needed in many different fields, such as in manufacturing of parts for automobiles, consumer electric products and industrial devices, and many adhesive agents have been developed for this purpose. Some of these adhesives are known to be very excellent. For example, adhesives that exhibit their function at normal temperature or with heating are used to integrally join metals and synthetic resins and this joining method is at present a common method for integrating composites composed of metals and synthetic resins.
On the other hand, more rational joining methods that do not involve the use of an adhesive have been studied heretofore. An example is a method in which a high-strength engineering plastic is integrated with a light metal such as magnesium, aluminum, an alloy of these or an iron alloy such as stainless steel without any adhesive. For instance, there has been proposed a method in which a shaped metal that has undergone surface treatment is inserted into a metallic mold, a thermoplastic resin is injected into the mold and the two are integrated, that is, a method based on a technique for utilizing injection molding technology to join a resin and a metal as a different material. This technology will be referred to as “injection joining” hereinafter. A known method for injection joining is to perform injection molding using a polybutylene terephthalate resin (hereinafter referred to as PBT) or a polyphenylene sulfide (hereinafter referred to as PPS) on an aluminum alloy (see Japanese Patent Application Laid-open 2004-216425: Patent Document 1, for example). Another known joining technique is to form relatively large holes in an anodized film on a piece of aluminum and make a synthetic resin protrude into the holes thus causing joint to be attained there (see WO/2004-055248 A1: Patent Document 2, for example).
The principle behind this injection joining in Patent Document 1 can be considered as follows. An aluminum alloy is immersed in a dilute aqueous solution of a water-soluble amine compound, the aluminum alloy is finely etched with a weakly basic aqueous solution and at the same time the amine compound molecules are adsorbed to the surface of the aluminum alloy. After undergoing this treatment, the aluminum alloy is inserted into a metallic mold for injection molding and a molten thermoplastic resin is injected under high pressure.
Here, the amine compound molecules adsorbed to the surface of the aluminum alloy encounter the thermoplastic resin to generate heat and the resin, which was apt to be quenched and solidified by contact with the aluminum alloy held at a low temperature of the mold, is not solidified as quickly and gets into ultrafine recesses on the aluminum alloy surface. As a result of this, with a composite composed of an aluminum alloy and a thermoplastic resin, the thermoplastic resin is securely joined without being separated from the aluminum alloy surface. That is, when an exothermic reaction occurs, a strong injection joining is produced. It has actually been confirmed that PBT and PPS, which can be exothermically reacted with an amine compound, can be joined by injection joining to an aluminum alloy.
The inventors have developed a resin composition that is suited to injection joining in order to make the above-mentioned technologies more effective and, specifically, made a further advancement and improvement on a technique of forming countless fine recesses in a metal surface prior to bonding. As a result, it was confirmed that a composition whose properties related to the crystallinity of PPS have been changed is particularly effective, rather than a simple PPS composition obtained merely by matching the linear coefficient of expansion to that of an aluminum alloy. That is, the inventors investigated whether the limitations of the pretreatment method required for metal parts could be reduced by making an advancement on the above-mentioned technologies and improving the resin composition parts and, as a result, discovered that the joint strength between a metal part and a resin composition part is further raised in a composite composed of a metal part and a resin composition part having a specific PPS composition, PBT composition or polyamide composition. The inventors found that, using this improved resin composition group, injection joining can be obtained not only with aluminum alloys but also with magnesium alloys, for example. The present invention relates to a technique for the injection joining of a hard resin to a magnesium alloy.
Let us touch briefly on the characteristics of magnesium alloys. The most prominent feature of magnesium alloys is that they are the lightest in weight of all practical metals and have a specific gravity close to 1.7, which is of lower value even compared to aluminum alloys (specific gravity of 2.7) touted for being so lightweight. On the other hand, however, they are more difficult to handle than aluminum alloys because they are far more chemically active. Specifically, with a magnesium alloy, immediately after a bare metal surface has been exposed by grinding or the like, oxygen in the air starts forming a natural oxidized layer and furthermore the stability and strength of this natural oxidized layer are markedly inferior to those of aluminum alloys. The natural oxidized layer of an aluminum alloy is known to be stable so that, as long as there is a rust preventing oil film or a paint film over this natural oxidized layer, the layer will remain stable for ten years or longer when left indoors where there is no condensation or the like. However, a magnesium alloy in the same environment will begin to swell or rust in less than a year. Molecules of carbon dioxide or water, diffusing through and penetrating the oil or paint film, pass through them reacting with the natural oxidized layer of the magnesium.
In other words, when a magnesium alloy is actually used, it has to be covered first with a tough film taking the place of a natural oxidized film. More specifically, the magnesium alloy is treated by chemical conversion treatment or electrolytic oxidation. The inventors confirmed at first that a specific resin composition could be joined by injection joining to a magnesium alloy that had undergone chemical conversion treatment but the result was still not good enough in all respects to be put to practical and commercial application. Specifically, there was such a problem that a chemical conversion treatment that would yield the best joint strength by injection joining does not have so good corrosion resistance as the product of the chemical conversion treatments currently in use.